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**Event-related fMRI (er-fMRI)**

Klaas Enno Stephan Translational Neuromodeling Unit (TNU) Institute for Biomedical Engineering, University of Zurich & ETH Zurich Laboratory for Social & Neural Systems Research (SNS), University of Zurich Wellcome Trust Centre for Neuroimaging, University College London With many thanks for slides & images to: FIL Methods group, particularly Rik Henson and Christian Ruff Methods & models for fMRI data analysis November 2012

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**Overview of SPM p <0.05 Statistical parametric map (SPM)**

Image time-series Kernel Design matrix Realignment Smoothing General linear model Statistical inference Gaussian field theory Normalisation p <0.05 Template Parameter estimates

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**Overview 1. Advantages of er-fMRI 2. BOLD impulse response**

3. General Linear Model 4. Temporal basis functions 5. Timing issues 6. Design optimisation 7. Nonlinearities at short SOAs

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Advantages of er-fMRI Randomised trial order c.f. confounds of blocked designs

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**er-fMRI: Stimulus randomisation**

Blocked designs may trigger expectations and cognitive sets … Unpleasant (U) Pleasant (P) Intermixed designs can minimise this by stimulus randomisation … … … … … Pleasant (P) Unpleasant (U) Unpleasant (U) Pleasant (P) Unpleasant (U)

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Advantages of er-fMRI Randomised trial order c.f. confounds of blocked designs Post hoc classification of trials e.g. according to performance or subsequent memory

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**er-fMRI: post-hoc classification of trials**

Participant response: „was not shown as picture“ „was shown as picture“ Items with wrong memory of picture („hat“) were associated with more occipital activity at encoding than items with correct rejection („brain“) Gonsalves & Paller (2000) Nature Neuroscience

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Advantages of er-fMRI Randomised trial order c.f. confounds of blocked designs Post hoc classification of trials e.g. according to performance or subsequent memory Some events can only be indicated by the subject e.g. spontaneous perceptual changes

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**eFMRI: “on-line” event-definition**

Bistable percepts Binocular rivalry

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Advantages of er-fMRI Randomised trial order c.f. confounds of blocked designs Post hoc classification of trials e.g. according to performance or subsequent memory Some events can only be indicated by the subject e.g. spontaneous perceptual changes Some trials cannot be blocked e.g. “oddball” designs

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**er-fMRI: “oddball” designs**

… time

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Advantages of er-fMRI Randomised trial order c.f. confounds of blocked designs Post hoc classification of trials e.g. according to performance or subsequent memory Some events can only be indicated by subject e.g. spontaneous perceptual changes Some trials cannot be blocked e.g. “oddball” designs More accurate models even for blocked designs? “state-item” interactions

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**er-fMRI: “event-based” model of block-designs**

“Epoch” model assumes constant neural processes throughout block U1 U2 U3 P1 P2 P3 “Event” model may capture state-item interactions Data U1 U2 U3 P1 P2 P3 Model

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**Modeling block designs: epochs vs events**

Designs can be blocked or intermixed, BUT models for blocked designs can be epoch- or event-related Epochs are periods of sustained stimulation (e.g, box-car functions) Events are impulses (delta-functions) Near-identical regressors can be created by 1) sustained epochs, 2) rapid series of events (SOAs<~3s) In SPM, all conditions are specified in terms of their 1) onsets and 2) durations … epochs: variable or constant duration … events: zero duration Sustained epoch “Classic” Boxcar function Series of events Delta functions Convolved with HRF

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**Disadvantages of er-fMRI**

Less efficient for detecting effects than blocked designs. Some psychological processes may be better blocked (e.g. task-switching, attentional instructions).

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BOLD impulse response Function of blood volume and deoxyhemoglobin content (Buxton et al. 1998) Peak (max. oxygenation) 4-6s post-stimulus; return to baseline after 20-30s initial undershoot sometimes observed (Malonek & Grinvald, 1996) Similar across V1, A1, S1… … but differences across other regions (Schacter et al. 1997) and individuals (Aguirre et al. 1998) Brief Stimulus Undershoot Initial Peak

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BOLD impulse response Early er-fMRI studies used a long Stimulus Onset Asynchrony (SOA) to allow BOLD response to return to baseline. However, if the BOLD response is explicitly modelled, overlap between successive responses at short SOAs can be accommodated… … particularly if responses are assumed to superpose linearly. Short SOAs can give a more efficient design (see below). Brief Stimulus Undershoot Initial Peak

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**General Linear (Convolution) Model**

For block designs, the exact shape of the convolution kernel (i.e. HRF) does not matter much. For event-related designs this becomes much more important. Usually, we use more than a single basis function to model the HRF. GLM for a single voxel: y(t) = u(t) h(t) + (t) u(t) h(t)= ßi fi (t) T 2T 3T ... convolution sampled each scan Design Matrix

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**Temporal basis functions**

Finite Impulse Response (FIR) model Fourier basis set Gamma functions set Informed basis set

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**Informed basis set Canonical Temporal Dispersion**

Canonical HRF (2 gamma functions) plus Multivariate Taylor expansion in: time (Temporal Derivative) width (Dispersion Derivative) F-tests allow testing for any responses of any shape. T-tests on canonical HRF alone (at 1st level) can be improved by derivatives reducing residual error, and can be interpreted as “amplitude” differences, assuming canonical HRF is a reasonable fit. Canonical Temporal Dispersion Friston et al. 1998, NeuroImage

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**Temporal basis sets: Which one?**

In this example (rapid motor response to faces, Henson et al, 2001)… Canonical + Temporal + Dispersion + FIR canonical + temporal + dispersion derivatives appear sufficient may not be for more complex trials (e.g. stimulus-delay-response) but then such trials better modelled with separate neural components (i.e. activity no longer delta function) (Zarahn, 1999)

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left occipital cortex right occipital cortex Penny et al. 2007, Hum. Brain Mapp.

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**Timing Issues : Practical**

TR=4s Scans Assume TR is 4s Sampling at [0,4,8,12…] post- stimulus may miss peak signal Stimulus (synchronous) Sampling rate=4s SOA = Stimulus onset asynchrony (= time between onsets of two subsequent stimuli)

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**Timing Issues : Practical**

TR=4s Scans Assume TR is 4s Sampling at [0,4,8,12…] post- stimulus may miss peak signal Higher effective sampling by: 1. Asynchrony, e.g. SOA = 1.5TR Stimulus (asynchronous) Sampling rate=2s SOA = Stimulus onset asynchrony (= time between onsets of two subsequent stimuli)

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**Timing Issues : Practical**

TR=4s Scans Assume TR is 4s Sampling at [0,4,8,12…] post- stimulus may miss peak signal Higher effective sampling by: 1. Asynchrony, e.g. SOA = 1.5TR 2. Random jitter, e.g. SOA = (2 ± 0.5)TR Better response characterisation (Miezin et al, 2000) Stimulus (random jitter) Sampling rate=2s SOA = Stimulus onset asynchrony (= time between onsets of two subsequent stimuli)

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Slice-timing T=16, TR=2s Scan 1 o T0=16 o T0=9

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Slice-timing Top slice Bottom slice Slices acquired at different times, yet model is the same for all slices => different results (using canonical HRF) for different reference slices Solutions: 1. Temporal interpolation of data … but less good for longer TRs 2. More general basis set (e.g. with temporal derivatives) … but more complicated design matrix TR=3s SPM{t} SPM{t} Interpolated SPM{t} Derivative Henson et al. 1999 SPM{F}

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**NB: efficiency depends on design matrix and the chosen contrast !**

Design efficiency The aim is to minimize the standard error of a t-contrast (i.e. the denominator of a t-statistic). This is equivalent to maximizing the efficiency ε: Noise variance Design variance If we assume that the noise variance is independent of the specific design: NB: efficiency depends on design matrix and the chosen contrast ! This is a relative measure: all we can say is that one design is more efficient than another (for a given contrast).

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** = Fixed SOA = 16s Not particularly efficient… Stimulus (“Neural”)**

HRF Predicted Data = Not particularly efficient…

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** = Fixed SOA = 4s Very inefficient… Stimulus (“Neural”) HRF**

Predicted Data = Very inefficient…

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** = Randomised, SOAmin= 4s More efficient … Stimulus (“Neural”) HRF**

Predicted Data = More efficient …

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** = Blocked, SOAmin= 4s Even more efficient… Stimulus (“Neural”) HRF**

Predicted Data = Even more efficient…

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**Another perspective on efficiency**

Hemodynamic transfer function (based on canonical HRF): neural activity (Hz) → BOLD Highpass-filtered efficiency = bandpassed signal energy Josephs & Henson 1999, Phil Trans B

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** = = Blocked, epoch = 20s Blocked-epoch (with short SOA)**

Stimulus (“Neural”) HRF Predicted Data = = Blocked-epoch (with short SOA)

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**Sinusoidal modulation, f = 1/33s**

Stimulus (“Neural”) HRF Predicted Data = = The most efficient design of all!

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**Blocked (80s), SOAmin=4s, highpass filter = 1/120s**

Predicted data (incl. HP filtering!) Stimulus (“Neural”) HRF = = Don’t use long (>60s) blocks!

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**Randomised, SOAmin=4s, highpass filter = 1/120s**

Stimulus (“Neural”) HRF Predicted Data = = Randomised design spreads power over frequencies.

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**Efficiency: Multiple event types**

Design parametrised by: SOAmin Minimum SOA pi(h) Probability of event-type i given history h of last m events With n event-types pi(h) is a nm n Transition Matrix Example: Randomised AB A B A B => ABBBABAABABAAA... 4s smoothing; 1/60s highpass filtering Differential Effect (A-B) Common Effect (A+B)

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**Efficiency: Multiple event types**

4s smoothing; 1/60s highpass filtering Example: Null events A B A B => AB-BAA--B---ABB... Efficient for differential and main effects at short SOA Equivalent to stochastic SOA (null event corresponds to a third unmodelled event-type) Null Events (A-B) Null Events (A+B)

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**Efficiency – main conclusions**

Optimal design for one contrast may not be optimal for another. Generally, blocked designs with short SOAs are the most efficient design. With randomised designs, optimal SOA for differential effect (A-B) is minimal SOA (assuming no saturation), whereas optimal SOA for common effect (A+B) is 16-20s. Inclusion of null events gives good efficiency for both common and differential effects at short SOAs.

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**But beware: Nonlinearities at short SOAs**

stim. presented alone stim. when preceded by another stim. (1 s) FIG. 1. Top panel: Simulated responses to a pair of words (bars) one second apart, presented together (solid line) and separately (broken lines) based on the kernels shown in Fig. 4. Lower panel: The response to the second word when presented alone (broken line as above) and when preceded by the first (solid line). The latter obtains by subtracting the response to the first word from the response to both. The difference reflects the effect of the first word on the response to the second. Friston et al. 2000, NeuroImage Friston et al. 1998, Magn. Res. Med.

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Thank you

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